Coherent fiber bundle parallel optical links

ABSTRACT

A coherent fiber bundle may be used to optically connect an array of microLEDs to an array of photodetectors in an optical communication system.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of the filing date of U.S.Provisional Patent Application No. 63/033,170 filed on Jun. 1, 2020, thedisclosure of which is incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates generally to generally to opticalcommunications using microLEDs, and more particularly to opticalcommunication systems using microLEDs and fiber bundles.

BACKGROUND OF THE INVENTION

Computing and networking performance requirements are ever increasing.Prominent applications include data center servers, high-performancecomputing clusters, artificial neural networks, and network switches.

For decades, dramatic integrated circuit (IC) performance and costimprovements were driven by shrinking transistor dimensions combinedwith increasing die sizes, summarized in the famous Moore's Law.Transistor counts in the billions have allowed consolidation onto asingle system-on-a-chip (SoC) of functionality that was previouslyfragmented across multiple ICs.

However, the benefits of further transistor shrinks are decreasingdramatically as decreasing marginal performance benefits combine withdecreased yields and increased per-transistor costs. Independent ofthese limitations, a single IC can only contain so much functionality,and that functionality is constrained because the IC's process cannot besimultaneously optimized for different functionality, e.g. logic, DRAM,and I/O. In fact, there are significant benefits to “de-integrating”SoCs into smaller “chiplets”, including: the process for each chipletcan be optimized to its function, e.g. logic, DRAM, high-speed I/O,etc.; chiplets are well-suited to reuse in multiple designs; chipletsare less expensive to design; chiplets have higher yield because theyare smaller with fewer devices.

However, a major drawback to chiplets compared to SoCs is that use ofchiplets generally requires far more chip-to-chip connections. Comparedto the on-chip connections between functional blocks in SoCs,chip-to-chip connections are typically much less dense and require farmore power (for example normalized as energy per bit).

Coupling optical sources and detectors to waveguides (including fibers)frequently dominates the cost of optical links and limits their density.

BRIEF SUMMARY OF THE INVENTION

Optical interconnects based on microLED (uLED) sources may offer a wayto overcome some or all of these limitations. A microLED may begenerally defined as a LED with a diameter of <100 um in someembodiments, <20 um in some embodiments, <4 um in some embodiments, and<1 um in some embodiments, and can be made with diameters <1 um. In someembodiments the uLED sources can support optical links with lengthsof >1 m at >1 Gbps with lower drive power and very high density.

Coherent fiber bundles (CFBs) comprised of large numbers of tightlypacked fibers are typically used in imaging and illuminationapplications. However, their unique properties are well-suited toovercoming cost and density issues in short, highly parallel opticallinks for chip-to-chip interconnects. In some embodiments a coherentfiber bundle provides an optical link for data communications between anoptical transmitter and an optical receiver. In some embodiments theoptical transmitter includes an LED as a light source, in someembodiments the LED is a uLED. In some embodiments the optical receivercomprises a photodetector. In some embodiments the uLED, coherent fiberbundle, and photodetector are within a same package, which is termed amultichip module. In some embodiments the multichip module includes aplurality of semiconductor chips on a common substrate. In someembodiments the optical transmitter is of a first chip of the pluralityof semiconductor chips. In some embodiments the optical receiver is of asecond chip of the plurality of semiconductor chips. In some embodimentsthe optical transmitter is of a first optical transceiver associatedwith a first chip of the plurality of semiconductor chips. In someembodiments the optical receiver is of a second optical transceiverassociated with a second chip of the plurality of semiconductor chips.

Some aspects provide an optical link for data communications betweenoptical transmitters and optical receivers, comprising: an opticaltransmitter array including a plurality of optical transmittersub-arrays, each optical transmitter sub-array of the plurality ofoptical transmitter sub-arrays comprising a plurality of microLEDs; afiber bundle comprised of a plurality of sub-bundles, each sub-bundlecomprised of a plurality of multimode fibers, each fiber including acore, the cores of each sub-bundle of the plurality of sub-bundles toreceive light from a corresponding one of the optical transmittersub-arrays; and an optical receiver array including a plurality ofoptical receiver sub-arrays, each optical receiver sub-array of theplurality of optical receiver sub-arrays comprising a plurality ofphotodetectors, each optical receiver sub-array of the plurality ofoptical receiver sub-arrays to receive light from cores of the fiberbundle.

In some aspects the optical transmitter array is of a first opticaltransceiver associated with a first semiconductor chip. In some aspectsthe optical receiver array of a second optical transceiver associatedwith a second semiconductor chip. In some aspects coherence ismaintained between fibers within each sub-bundle. In some aspectscoherence is not maintained between fibers in different sub-bundles. Insome aspects relative positions of outputs of fibers in each sub-bundleof the plurality of sub-bundles are the same as relative positionsinputs of fibers in each sub-bundle. In some aspects the microLEDs areattached to a first substrate. In some aspects the microLEDs areattached to an IC that also includes microLED drive circuitry. In someaspects the photodetectors are mounted to a second substrate. In someaspects the photodetectors are monolithically integrated with receivercircuits on an IC. In some aspects light from each microLED is coupledinto only a single core. In some aspects each photodetector is toreceive light from only a single core. In some aspects light from eachmicroLED is coupled into multiple cores. In some aspects a plurality ofphotodetectors are to receive light generated by each microLED. Someaspects further comprise at least one turning mirror to direct lightfrom the microLEDs towards inputs of the fiber bundle. Some aspectsfurther comprise a turning mirror for each optical transmittersub-array, to direct light from microLEDs of the optical transmittersub-array towards inputs of the sub-bundle associated with the opticaltransmitter sub-array. Some aspects further comprise an input arrayoptical coupling assembly coupling the optical transmitter array and thefiber bundle. In some aspects the input array optical coupling assemblycomprises a forty-five degree turning mirror such that light incident onthe mirror is reflected at a ninety degree angle, a first lenspositioned between the turning mirror and the microLEDs, and a secondlens positioned between the turning mirror and the fiber bundle.

These and other aspects of the invention are more fully comprehendedupon review of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of an embodiment of a coherent fiberbundle.

FIG. 2 shows a cross-sectional view of a coherent fiber bundle withspatially-sampled optical distribution on an input face of the coherentfiber bundle and at an output face of the coherent fiber bundle.

FIG. 3 shows a parallel optical link using a coherent fiber bundle.

FIG. 4 a is a block diagram of an example transmitter.

FIG. 4 b shows an embodiment of LED collection optics comprising a lensand parabolic mirrors.

FIG. 4 c is a block diagram of an example receiver.

FIG. 5 a shows a single turning mirror placed between two lenses to turna beam 90°.

FIG. 5 b shows the use of two turning mirrors to turn the beam 180°.

FIGS. 6 a-c show photodetectors arranged in a grid pattern, with aplurality of “spots” illuminated with light.

FIG. 7 a shows photodetectors switchably connectable to a transimpedanceamplifier.

FIG. 7 b shows photodetectors with switched connections to neighborscreating optimized composite photodetectors for two spots.

FIG. 8 shows transmitter and receiver sub-arrays coupled by coherentfiber sub-bundles.

DETAILED DESCRIPTION

A cross-section of an embodiment of a coherent fiber bundle (CFB) 109 isshown in FIG. 1 . The coherent fiber bundle includes a plurality ofcores 111 for the transmission of light. Each core is surrounded by aconcentric cladding layer 113 that has a lower index of refraction thanthe core. Light is guided in the cores; each core plus cladding may bereferred to as a “fiber.” Multimode fibers are preferred for use withmicroLED sources as multimode fibers allow for greater coupling of lightfrom microLEDs than for single-mode fiber. The numerical aperture (NA)of a fiber is defined as NA=sin(θ_(c)), where θ_(c) is the maximumexternal acceptance angle of the fiber (relative to the fiber'spropagation axis); rays at angles larger than θ_(c) are not guided bythe fiber.

A large number of fibers may be grouped together in a “bundle.” In someembodiments, the fibers may be arranged in a regular pattern, such as ona square grid 117 or hexagonal grid 119.

In other embodiments, cores of multiple sizes may be used to improvepacking density. Some kind of jacket material 115 such as a polymer orglass may be used in the interstitial areas between the fibers to holdthe bundled fibers together. In some embodiments, the jacket material isthe same as the cladding material. In some embodiments, the jacketmaterial may be highly optically absorbing to attenuate any light notpropagating in a core.

The bundle is referred to as “coherent” if the relative positions of thefibers are the same at the fiber output as at the fiber input, e.g. thefibers do not cross over each other so the relative input and outputpositions of the fibers are preserved. A CFB reproduces aspatially-sampled version of the optical distribution on its input faceat its output face, where the spatial sampling resolution is equal tothe core-to-core spacing. FIG. 2 shows a cross-sectional view of acoherent fiber bundle with spatially-sampled optical distribution on aninput face of the coherent fiber bundle and at an output face of thecoherent fiber. In FIG. 2 , for the CFB 211, a plurality of spots 221illuminate cores within the spots. A first exploded view shows a portionof an input face 231 of the CFB, with cores 213, arranged in a squaregrid pattern, and the spot illuminating some of the cores. A secondexploded view shows a portion of an output face 233 of the CFB, againwith cores 213, arranged in the same pattern, and the spot illuminatingthose same cores.

FIG. 3 shows a parallel optical link using a coherent fiber bundle. Anoptical transmitter array 311 is coupled to the CFB 315 by an inputarray optical coupling assembly (IAOCA) 313. An output of the CFB iscoupled to a receiver array 319 by an output array optical couplingassembly (OAOCA) 317.

The optical transmitter array comprises an array of individualtransmitters 321. A block diagram of an example transmitter is shown inFIG. 4 a . A transmitter comprises a microLED drive circuit 411. Thedrive circuit is to receive an input electrical signal, and drive amicroLED 413. The microLED outputs an optical signal modulated by theinput electrical signal. In some set of embodiments, the drive circuitcomprises equalization circuitry that increases the maximum data ratesupported by the transmitter. In a set of embodiments, the electricaland optical signals utilize non-return-to-zero (NRZ) modulation. Inother embodiments, other modulation formats may be used, such as PAM-Nand N-QAM, where N is a power of two. The transmitter of FIG. 4 a isshown as including LED collection optics 415. In some embodiments theLED collection optics may not be considered as part of the transmitter,with instead the LED collection optics being part of the input arrayoptical coupling assembly of FIG. 3 , for example.

In some sets of embodiments, LED collection optics are used to collectthe light emitted by the microLED. The LED collection optics maycomprise an encapsulant layer, lenses, flat mirrors, and/or curvedmirrors. FIG. 4 b shows an embodiment of LED collection opticscomprising a lens 421 and mirrors or reflectors 423, which may becurved, for example parabolic, or flat. These optics collect the lightfrom the microLED 413 and reduce the angular cone of the opticaldistribution compared to that emitted directly from the microLED.

In some sets of embodiments, the microLEDs in a transmitter array areall attached to a single substrate 425. The substrate may be made fromvarious materials including sapphire, silicon, glass, ceramic, or thesame semiconductor material as the microLED, e.g. GaN, GaAs, or InP.

A receiver array comprises an array of individual receivers. A blockdiagram of an example receiver is shown in FIG. 4 c . The receivercomprises one or more photodetectors (PDs) 431 and receiver electronics433. In a set of embodiments, the receiver also includes PD collectionoptics 435 placed in front of the PD(s), although in some embodimentsthe PD collection optics may be part of an output array optical couplingassembly (with output referring to an output of a fiber bundle, forexample). The PD collection optics may comprise one or more lenses andspatial filtering elements. In a set of embodiments, a receiver containsone photodetector (PD). In another set of embodiments, a receivercontains more than one PD. Each PD converts an input optical signal toan output electrical signal. The electrical output signals from the PDor PDs are connected to the receiver circuit that amplifies the signal.In some embodiments, the receiver circuit also comprises an equalizer, adecision circuit, a limiting amplifier, and/or buffer amplifiers.

In some sets of embodiments, all of the PDs in the array are mounted toa single substrate. The substrate may be made from various materialsincluding sapphire, silicon, glass, ceramic, or the same semiconductormaterial as the PD, e.g. Si, Ge, or InGaAs. In some sets of embodiments,the circuitry for some or all of the receivers are integrated onto acommon IC. In some sets of embodiments, the PDs are monolithicallyintegrated with the receiver circuits. This is especially useful formaterials in which high-performance, high-density, low-cost electronicsare available such as silicon.

Referring again to FIGS. 2 and 3 , the input optical coupling assemblycouples the light from each optical transmitter to form an opticaldistribution (a “spot”) on the input face of the CFB. In a set ofembodiments, the spot from each transmitter is small enough and alignedin a way that it is coupled into only one core of the CFB. In anotherset of embodiments, the light from each transmitter forms a larger spotsuch that it is coupled into N cores of the CFB, for example as shown inFIG. 2 . In some of these embodiments, there is no significant overlapbetween the input optical spots from different transmitters so that agiven core carries light from only a single optical source, or does notcarry light at all. In other embodiments, there is some overlap betweenthe optical spots at the input face of the CFB so that some cores carrylight from two or more transmitters.

In a set of embodiments, the transmitter array may be arranged in aregular pattern, e.g. a square, rectangular, or hexagonal grid.Similarly, the fibers in a CFB may be arranged in some regular pattern.In embodiments where a core carries light from a single transmitter (orno light at all), the pattern of the cores carrying light matches thatof the transmitter array. In a further set of embodiments, optics withmagnification M are interposed between the transmitter array and theCFB; in these embodiments, the CFB core pattern is the same as that ofthe transmitter array but magnified by M.

In embodiments where light from a single transmitter is coupled tomultiple cores, the CFB grid may not match that of the transmitterarray. In a set of embodiments, neither the transmitter array nor thecores is on a regular grid. As the number of cores per transmitter spotincreases, there is less variation in optical link loss as an inputoptical spot is moved across the input face of the CFB. Compared tocoupling each transmitter to a single core, the embodiments where eachtransmitter is coupled to multiple cores provide benefits including: a)Non-critical optical source-to-fiber alignment: Each spot that fallsanywhere on the input face of the CFB will propagate to the output endof the CFB; b) Support for various transmitted spot sizes: Differentinput spot sizes merely illuminate different numbers of cores; c)Support for various transmitted spot spacing: The center-to-centerspacing of spots is flexible as long as the spots are incident on theCFB face—if the gap between spots is greater than one core diameter, agiven core will carry light from only a single source; d) Support forvarious numbers of transmitted spots: Again, any input spot distributionis supported as long as all spots fall on the CFB's input face.

The output light from the transmitter array is relayed to the input faceof the CFB by the IAOCA, for example the IAOCA of FIG. 3 , whichcomprises one, some, or all of lenses, flat mirrors, and/or curvedmirrors. In a set of embodiments, the IAOCA may magnify the opticaldistribution from the transmitter array by a magnification M, where thespot size and center-to-center spot spacing scales as M while theangular spread from each LED scales as 1/M. This allows the angularspread to be matched to the NA of the CFB cores to achieve high couplingefficiency. In some embodiments, the IAOCA may be omitted, with forexample the transmitter array butt-coupled to the CFB.

In a set of embodiments, the IAOCA comprises one or more turningmirrors, allowing the beam direction to be changed. This may allowCFB-based optical links to fit within tight space constraints. FIG. 5 ashows a single turning mirror placed between two lenses to turn the beam90°. As illustrated in FIG. 5 a , an array of microLEDs 511, with theircollection optics, is positioned below an input array optical couplingassembly (IAOCA) 513. The IAOCA includes a first lens 515 a, a turningmirror 517, and a second lens 515 b. The first lens is positionedbetween the microLEDs and the turning mirror. The turning mirror isangled at 45° relative to vertical and horizontal planes, so as toredirect light from a vertical direction to a horizontal direction. Thesecond lens is positioned between the turning mirror and a coherentfiber bundle (CFB) 519. The CFB is shown as being to a side of theIAOCA. FIG. 5 b shows the use of two turning mirrors to turn the beam180°. As in FIG. 5 a , in FIG. 5 b an array of microLEDs 521, with theircollection optics, is positioned below an input array optical couplingassembly (IAOCA) 523. The IAOCA of FIG. 5 b includes a first lens 525 a,a first turning mirror 527 a, a second turning mirror 527 b, and asecond lens 515 b. The first lens is positioned between the microLEDsand the first turning mirror. The first turning mirror is angled at 45°relative to vertical and horizontal planes, so as to redirect light froma vertical direction to a horizontal direction. The second lens ispositioned to direct light from the from the first turning mirror backto the vertical direction. The second lens is positioned between thesecond turning mirror and a coherent fiber bundle (CFB) 529. The CFB isshown as being below the IAOCA.

It is generally desirable that the CFB fibers have a high NA becausethat allows them to more efficiently capture light from microLEDassemblies with a high angular spread. There are typically practicallimits to the fiber NA based on material properties. The opticalmagnification techniques discussed above can match the angular spread ofthe transmitter array to the NA of the CFB cores by increasing the spotsize from each microLED.

From an optical loss perspective, it is advantageous to maximize theratio of core area to cladding area, since light launched into thecladding and jacket is lost. However, the trade-off is that core-to-coreevanescent crosstalk increases as the core-to-core distance isdecreased. The minimum distance between the cores that can be toleratedis generally a function of the link length, the number of cores betweenspots, and the core NA.

Another source of crosstalk is light that does not propagate in thecores but instead propagates in the cladding or the jacket material. Ifthis light propagates to the output of the CFB, some of it may becoupled to the receiver array. Because this light is not guided by thecores, some of it may be from adjacent channels and contributeinter-channel crosstalk.

To reduce crosstalk due to this light, in some embodiments the jacketmaterial may be highly optically absorbing, for example at the signalwavelength. In some embodiments, this crosstalk may be reduced byemploying spatial filtering elements in the output optical couplingassembly and/or the PD collection optics such that only light carried inthe cores is coupled to the photodetectors.

Referring again to FIG. 3 , the output array optical coupling assembly(OAOCA) relays light from the output face of the CFB to the receiverarray. In a set of embodiments, the output optical coupling assembly maycomprise one of or a combination of lenses, flat mirrors, spatialfiltering elements, and curved mirrors, for example similar to or thesame as that of the IAOCA. The spatial filtering elements may becomprised of optically absorbing materials that for example may be usedto block light that did not travel in the CFB cores, thus reducinginter-channel crosstalk. In another set of embodiments, the OAOCA may beomitted, with for example the output face of the CFB butt-coupled to thereceiver array.

As noted previously, if the light for each input signal channel iscoupled into one or more CFB cores, the light propagates to the endfaceof the CFB to create an output optical “spot” that is aspatially-sampled version of the input optical spot. Each CFB outputspot is relayed by the output optics to a corresponding “spot” in the PDplane; again, “spot” in the PD plane is an optical distributionassociated with a single transmitter channel. In some embodiments, eachPD plane spot falls on a single PD. Each PD may be larger than the spotin the PD plane, which may increase alignment tolerances. Thedisadvantage of increasing PD size is that it tends to increase PDcapacitance, and may also pick up more inter-channel optical crosstalk.

In another set of embodiments, each PD plane spot may illuminatemultiple PDs as shown in FIGS. 6 a -c. FIGS. 6 a-c show PDs arranged ina grid pattern, with a plurality of “spots” illuminated with light. ForFIG. 6 a , each of nine spots, e.g. spot 911 a, covers, in whole or inpart, a 3×3 set of PDs, e.g, PD 913 a. The spots in FIG. 6 a do notoverlap, with at least one PD present between spots. FIG. 6 b is similarto FIG. 6 a , with nine spots, e.g. spot 911 b, each cover a pluralityof PDs, e.g. PD 913 b. The spots in FIG. 6 b , however, partiallyoverlap, with for example a central spot being partially overlapped byfour other spots. FIG. 6 c shows even greater overlap of spots, with forexample a central spot 911 c overlapped by eight adjacent spots. InFIGS. 6 a -c, the PD plane comprises a regular grid of PDs. In a set ofembodiments, the PDs are interconnected in some pattern with electricalswitches. In some of these embodiments, an electrical switch comprisessome type of transistor such as a field-effect transistor (FET).

If the switch between two PDs is in the “on” (low resistance) state, thecurrents from the PDs are summed. A set of PDs connected by “on”switches can be thought of as a larger “composite PD.” By changing thestate of the electrical switches, the location and size of eachcomposite PD can be changed. Each composite PD is connected to atransimpedance amplifier (TIA) and subsequent receiver electronics. Theconnection between a composite PD and a TIA may comprise one or more“on” switches.

The signal from each channel can be recovered by causing the locationand size of a composite PD to coincide with that channel's PD planeoptical spot. Conceptually, rather than aligning the spots with the PDs,the composite PDs “move” to where the spots are. This allows loosealignment tolerances between the output end of the CFB and thesubsequent optics (including the receiver array). By contrast, the useof conventional fixed PD locations often requires translationalalignment accuracies of a few microns. The greatly reduced alignmenttolerances enabled by a switched PD grid may significantly decreasepackaging costs and increase yields relative to a conventional fixed PDarrangement.

In a set of embodiments, analog switches, or analog componentsassociated with digital switches, connect the PDs in a manner such thatthe output current from each composite PD is the weighted sum of theindividual PD currents.

For a given optical distribution on the PD plane, an algorithm can beused to determine how the PDs should be interconnected to maximize thesignal from each channel while minimizing inter-channel crosstalk. Thisis equivalent to moving and resizing the composite PDs.

The algorithm can be implemented, in some embodiments, with low-speedelectronics and/or software to measure the current generated from eachPD with a training optical distribution on the receiver array. The lightfalling in each PD is the weighted sum of contributions from the PDplane spots for the various channels. If there are M channels and N PDs,the output current from the PDs can be expressed as:

I_(PD)=R_(w) P_(chan)   (Eqn. 1)

where I_(PD) is an N-element vector containing the currents from the NPDs, P_(chan) is an M-element vector containing the optical power fromeach of the M channels in the PD plane, and R_(w) is an N×M elementmatrix that expresses the effective responsivity between each channeland each PD.

The optical power from each channel can be recovered by inverting R_(w):

P_(chan) =R _(w) ⁻¹ I _(PD)   (Eqn. 2)

In some embodiments this computation is done during a training session.Assuming that the entire LED-to-PD optical link losses are stable overtime, in some embodiments this computation is done only during aninitial training session.

In a set of embodiments, the light falling on each PD is dominantlyassociated with a single channel, for example as shown in FIG. 6 a . Inthis case, the effective responsivity matrix R_(w) is diagonal and thematrix inversion of Equation (2) is trivial.

In another set of embodiments, each PD is overlapped by a maximum ofonly a few beams, for example as shown in FIG. 6 b ). In this case, theresponsitivity matrix R_(w) will be sparse, allowing sparse matrixcomputational methods to be used. In another set of embodiments, thereis large overlap between beams, for example as shown in FIG. 6 c . Inthis case, not only may the matrix inversion be computationallyintensive, but it may not be possible to obtain adequatesignal-to-crosstalk ratios to accurately recover the signal from eachchannel in some instances.

In the set of embodiments with no or little beam overlap, connectingeach PD to some subset of its neighbors may be adequate to optimize thecomposite pixel locations for all beam locations. FIG. 7 a shows PDs 711switchably connectable to a transimpedance amplifier 713. In FIG. 7 a ,twelve PDs are arranged in a 3×4 grid, with photodetectors of each rowcouplable by switches and photodetectors of the first and last column(third column) also couplable by switches, which may be open 715 orclosed 717. The PDs of the last row are also couplable to a TIA byswitches. FIG. 7 a shows a composite PD formed by closing switchescoupling the PDs of the middle rows together for the second and thirdcolumn PDs (as well as closing the switches coupling the PDs of thethird column for the PDs in the middle rows). The switches coupling themiddle row PDs of the third column are also closed, coupling thecomposite PD to the TIA. FIG. 7 b shows PDs with switched connections toneighbors creating optimized composite PDs for two PD plane spots. InFIG. 7 b , a plurality of PDs, e.g. PD 721, are arranged in arectangular grid. A first “spot” 723 a illuminates a first area of thegrid, and a second “spot” 723 b illuminates a second area of the grid.The PDs are couplable by switches. Switches between adjacent PDs atleast partially illuminated by the spots are closed 727, which switchesbetween adjacent PDs that are not both at least partially illuminated bythe spots are open 725. The result is a first composite PD 731 a isformed of PDs at least partially illuminated by the first spot, and asecond composite PD 731 b is formed of PDs at least partiallyilluminated by the second spot.

In a set of embodiments, the PD plane spots are of a known size,center-to-center spacing, and pattern (e.g., square or hexagonal grid).The “uncertainty” associated with the optical misalignments correspondto translations and rotations of this known pattern. Knowledge of thisgrid pattern can be used to simplify/optimize the connectivity of theswitches connecting the PDs, and to simplify computation of the optimalswitch states.

In the previous discussions of CFB-based links, a single micro-LED arrayhas been connected by a CFB to a single receiver array. This can begeneralized by dividing the transmitter and receiver arrays intosub-arrays and the CFB into sub-bundles. FIG. 8 shows the case of Minput assemblies coupled to N output assemblies. Each input and outputassembly has the same structure as those for the full bundle of FIG. 3 ,e.g., a transmitter sub-array 811 coupled to an IAOCA 813 on each inputsub-bundle 815 and an OAOCA 817 coupled to a receiver sub-array 819 oneach output sub-bundle. The sub-bundles are combined to form a bundle821.

Preferably the “coherence” is maintained within each sub-bundle, e.g.,the relative positions of the output fibers in a sub-bundle are the sameas that at the input. The exception to this is that relative fiberpositions are obviously not maintained between fibers in differentsub-bundles, in some embodiments.

A CFB can be divided into sub-bundles, for example, by removing some ofthe jacket material that binds a sub-bundle to the rest of the fibers.This can be done by a “leaching” process that utilizes a jacket solvent.

In some sets of embodiments, m=1 and n>1 so the connectivity is that ofa splitter. In another set of embodiments, m>1 and n=1 so theconnectivity is that of a combiner. In another set of embodiments, m>1and n>1 so the connectivity is that of a star coupler, if cores of inputsub-bundles are distributed amongst the output sub-bundles.

Although the invention has been discussed with respect to variousembodiments, it should be recognized that the invention comprises thenovel and non-obvious claims supported by this disclosure.

1. An optical link for data communications between optical transmittersand optical receivers, comprising: microLED array including a pluralityof microLED sub-arrays, each microLED sub-array of the plurality ofmicroLED sub-arrays comprising a plurality of microLEDs; a fiber bundlecomprised of a plurality of sub-bundles, each sub-bundle comprised of aplurality of multimode fibers, each fiber including a core, eachsub-bundle being a coherent fiber bundle, such that a spatially-sampledversion of an optical distribution on a first face of the coherent fiberbundle is reproduced on a second face of the coherent fiber bundle, withrelative positions of inputs of fibers the same as relative positions ofoutputs of fibers for fibers in each sub-bundle, but not for fibers indifferent sub-bundles, the cores of each sub-bundle of the plurality ofsub-bundles to receive light from a corresponding one of the microLEDsub-arrays; and photodetector array including a plurality ofphotodetector sub-arrays, each photodetector sub-array of the pluralityof photodetector sub-arrays comprising a plurality of photodetectors,each photodetector sub-array of the plurality of photodetectorsub-arrays to receive light from cores of the fiber bundle.
 2. Theoptical link of claim 1, wherein the microLED array is of a firstoptical transceiver associated with a first semiconductor chip.
 3. Theoptical link of claim 1, wherein the photodetector array of a secondoptical transceiver associated with a second semiconductor chip. 4.-6.(canceled)
 7. The optical link of claim 1, wherein the microLEDs areattached to a first substrate.
 8. The optical link of claim 1, whereinthe photodetectors are mounted to a second substrate.
 9. The opticallink of claim 1, wherein the microLEDs are mounted to an IC that alsoincludes microLED drive circuitry.
 10. The optical link of claim 1,wherein the photodetectors are monolithically integrated with receivercircuits.
 11. The optical link of claim 1, wherein light from eachmicroLED is coupled into only a single core.
 12. The optical link ofclaim 11, wherein each photodetector is to receive light from only asingle core.
 13. The optical link of claim 1, wherein light from eachmicroLED is coupled into multiple cores.
 14. The optical link of claim13, wherein a plurality of photodetectors are to receive light generatedby each microLED.
 15. The optical link of claim 1, further comprising atleast one turning mirror to direct light from the microLEDs towardsinputs of the fiber bundle.
 16. The optical link of claim 1, furthercomprising a turning mirror for each microLED sub-array, to direct lightfrom microLEDs of the microLED sub-array towards inputs of thesub-bundle associated with the microLED sub-array. 17.-18. (canceled)